1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for transmitting/receiving signals in a communication system.
2. Description of the Related Art
The use of terminals such as, for example, smartphones has led to an exponential increase in the average amount of data being used by mobile communication users as well as a constant increase in user demand for higher data transmission rate. In general, as a method of providing higher data transmission rate, there are methods of providing communications on wider frequency bands and methods of improving frequency usage efficiency. However, achieving sufficiently higher data transmission rates with the latter method is difficult because recent communication technologies have met limitations in improving the frequency usage efficiency through technical enhancements because such technologies have already been providing a frequency usage efficiency as close to a theoretical limit as possible. Thus, a feasible method of improving the data transmission rate may be focused on providing data services via wider frequency band. In this regard, available frequency bands must be considered when improving the data transmission rate by providing data services via a wider frequency band. Under the current frequency distribution policy, over a 1 GHz band available for wideband communications is limited and thus, selectable frequency bands in reality are only some ultra-high frequency bands (e.g., millimeter wave bands over 30 GHz). In contrast to cellular systems according to the related art that use 2 GHz bands, in systems that use ultra-high frequency bands, severe signal attenuation occurs over distance. The signal attenuation significantly reduces service coverage of a base station that uses the same power as a cellular system according to the related art. To solve this problem, a beamforming technique is widely used to increase transmission/reception efficiency of antennas by concentrating transmission/reception power within a narrow space.
FIG. 1 illustrates a base station and a terminal that provide beamforming using an array antenna according to the related art.
Referring to FIG. 1, a base station 110 may transmit data by alternating downlink transmission (TX) beam directions using a plurality of array antennas (Array0, Array 1) in each cell. A terminal 130 may also receive the data by alternating its reception (RX) beam directions.
In a system that communicates using the beamforming technique, the base station 110 and the terminal 130 provide data services by selecting transmitting and receiving beam directions that show or demonstrate an optimum channel condition from among various transmitting beam directions and receiving beam directions. This equally applies not only to downlink channels for data transmission from the base station 110 to the terminal 130 but also to uplink channels for data transmission from the terminal 130 to the base station 110. Assuming that there are N directions of the transmitting beam available to the base station 110 and M directions of the receiving beam available to the terminal 130, the simplest method of selecting the optimum downlink transmission/reception direction is for the base station 110 to transmit a predetermined signal more than at least M times in each of N transmission directions and for the terminal 130 to receive each of N transmitting beams using the M transmitting beams. According to this method, the base station 110 transmits a particular reference signal at least N×M times, and the terminal 130 receives the reference signal N×M times and measures reception strength of the received signal. The terminal 130 may determine a direction that shows or demonstrates the strongest measurement among the corresponding N×M measurements to be the optimum transmission/receiving beam direction. A process for the base station 110 to transmit a signal more than one time in all directions available for transmission is called a beam sweeping process, and a process for the terminal 130 to select an optimum transmission/receiving beam direction is called a beam selection process. The beam selection process may equally apply to a process of uplink data transmission/reception from the terminal 130 to the base station 110.
In cellular systems according to the related art, the base station 110 transmits common control channels, such as sync channels SCH or broadcast channels BCH all over the coverage of the base station. In a system that communicates using the beamforming technique, as shown in FIG. 1, in order to transmit the sync channels and the broadcast channels all over the coverage of the base station 110, the base station 110 transmits the channels in all directions available for transmission in the beam sweeping process. Frequency of transmission required to transmit the sync channel and the broadcast channel in the beam sweeping process is proportional to the number of transmitting beams present in the coverage of the base station. Thus, the simplest way of reducing transmission overhead of broadcast-type channels is to cover the entire coverage of the base station 110 with a fewer number of transmitting beams. In order to cover the entire coverage of the base station with a fewer number of transmitting beams, each transmitting beam should have a wide beam width. For example, to cover a sector 60 degrees wide with two transmitting beams, each of the two transmitting beams should be about 30 degrees wide.
However, as the beam width becomes wider, the beamforming effect is reduced in proportion to the beam width, and as the beam width becomes narrower, the beamforming effect is increased. If the beam width narrows to increase the beamforming effect, the number of transmitting beams required to cover the coverage of a single base station must increase accordingly, and thus the overhead for transmitting the broadcast-type channels also increases. Consequently, there is trade-off relation between the beamforming effect and the overhead for transmission of broadcast channels.
To solve this problem effectively, a scheme of diversifying the beam width used to transmit broadcast channels and the beam width used to transmit user data is used. For example, within a 60-degree sector, a transmitting beam 30 degrees wide may be used to transmit broadcast channels and a transmitting beam 10 degrees wide may be used to transmit the user data. In the scheme that uses a plurality of beam widths, a transmitting beam having a wide beam width is called a wide beam, and a transmitting beam having a narrow beam width is called a narrow beam or a fine beam.
The foregoing downlink beam sweeping and beam selection processes may be equally applied to a random access process in which the terminal first establishes a channel to transmit data to the base station.
FIG. 2 illustrates transmitting beams being transmitted by a terminal for uplink random access according to the related art.
Referring to FIG. 2, a number of transmitting beams is determined on the assumption that the base station has 4 receiving beam directions and the terminal has 4 transmitting beam directions.
The terminal transmits transmitting beams in all available directions toward each receiving beam of the base station. Similarly, the base station transmits transmitting beams in all available directions toward each receiving beam of the terminal. As illustrated in FIG. 2, the process is repeated for a total of 16 cycles. The base station then makes a plurality of attempts to detect random access information of the terminals with each uplink receiving beam (e.g., the base station attempts to detect a random access signal 4 times for each receiving beam, which corresponds to a total of 16 times). Accordingly, the base station may receive the random access information with transmitting and receiving beams that show or demonstrate the optimum reception performance.
In the meantime, during the random access procedure, the base station generally uses a beam having a beam width that is of a similar width as to downlink broadcast type channels as the receiving beam in order to reduce the foregoing beam sweeping overhead. However, for the uplink channel that requires maximum transmission power of the terminal, which is about 20 dB lower than the transmission power of the base station, a higher level of beamforming effect needs to be used than that of the downlink transmission channel. To achieve the higher level of beamforming effect, a receiving beam having a narrower beam width should be used to receive the random access signal. For example, if 4 receiving beams are replaced with 16 receiving beams having narrower beam widths, the base station has to attempt to detect the random access signal of the terminal 4 times for each receiving beam, which corresponds to a total of 64 (16×4) times that the base station has to attempt to detect the random access signal of the terminal. It is seen from the example that using the narrow receiving beam to receive the random access signal causes huge overhead relative to a system using the wide receiving beam.
Furthermore, to perform the foregoing random access operation, the terminal uses sufficient transmission power to cope with a path loss due to signal attenuation over distance between the base station and the terminal or attenuation effects caused from signal scattering and absorption at a mirror. In the communication system according to the related art, the terminal measured the path loss based on a difference between information about transmission intensity at the base station when the base station transmits a reference signal on the downlink communication channel and information about reception intensity at the terminal when the terminal receives the reference signal on the downlink communication channel. However, in case of the wideband communication system using the beamforming technique, in uplink random access, different path losses may occur depending on transmitting/receiving beam directions and transmitting/receiving beam widths, and thus disabling to use of existing path loss compensation techniques known to the communication system according to the related art.
Therefore, a need exists for a system and method for transmitting/receiving signals in a communication system that performs beamforming using a plurality of beam widths.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.